1. Field of the Invention
The present invention relates to a light emitting device having a plurality of light emitting elements, and an optical device using the same.
2. Description of the Related Art
In recent years, in the field of light emitting devices, a semiconductor laser (LD; laser diode) in which a plurality of light emitting portions of different output wavelengths are formed on the same substrate (or board) (hereinafter referred to as a multiple-wavelength laser) is actively developed. An example of such a multiple-wavelength laser is, as shown in FIG. 1, obtained by forming a plurality of light emitting portions of different output wavelengths on a single chip (what is called a monolithic type multiple-wavelength laser). In the multiple-wavelength laser, for example, a lasing portion 201 formed by growing layers of semiconductor materials of the system AlGaAs by vapor phase epitaxy and a lasing portion 202 formed by growing layers of semiconductor materials of the system AlGaInP are disposed side by side on one face of a substrate 212 made of GaAs (gallium arsenide) with an isolation groove 211 between them. In this case, the output wavelength of the lasing portion 201 is in the range of the order of 700 nm (for example, 780 nm) and that of the lasing portion 202 is in the range of the order of 600 nm (for example, 650 nm).
Except for the structure shown in FIG. 1, a structure (what is called a hybrid type multiple-wavelength laser) in which a plurality of semiconductor lasers LD1 and LD2 having different output wavelengths are mounted side by side on a board 221 has been also proposed. The above-mentioned monolithic-type laser is, however, more effective in controlling the light emitting point intervals with high accuracy.
These multiple-wavelength lasers are used, for example, as laser light sources of optical disk drives. At present, in an optical disk drive, semiconductor laser light in the range of the order of 700 nm is generally used for optical playback of CD (Compact Disk) recording or for optical recording/playback using recordable optical disks such as CD-Rs (recordable CDs), CD-RWs (rewritable CDs) or MDs (Mini Disks). Semiconductor laser light in the range of the order of 600 nm is used for optical recording/playback using DVDs (Digital Versatile Disks). By mounting a multiple-wavelength laser as described above on an optical disk drive, optical recording/playback becomes possible with respect to any existing optical disks. Moreover, the lasing portions 201 and 202 are disposed side by side on the same substrate (as for the semiconductor lasers LD1 and LD2 of the hybrid type, on the same board), only one package is necessary for the laser light source. The number of parts of an optical system such as an objective lens and a beam splitter for optical recording/playback using various optical disks is decreased to simplify the configuration of the optical system. Thus, reduction in size and cost of an optical disk drive can be achieved.
Meanwhile, in recent years, the demand for further growth of optical recording area density by using semiconductor lasers of shorter output wavelengths has been growing. Heretofore known materials of semiconductor lasers addressing the demand are Group III-V compound semiconductors of the nitride system (hereinbelow, also called semiconductors of the system GaN) typified by GaN, AlGaN mixed crystals, and GaInN mixed crystals. Semiconductor lasers using semiconductors of the system GaN are capable of light emission at a wavelength of around 400 nm, which is regarded as the limit wavelength at which optical recording/playback is done using an optical disk and an existing optical system, and therefore, they receive much attention as light sources of next-generation optical recording/playback apparatuses. It is also expected as light sources of full-color displays using three primary colors of RGB. Thus, development of multiple-wavelength lasers with lasing portions of the system GaN is desired.
As an example of related-art multiple-wavelength lasers with lasing portions of the system GaN, as shown in FIG. 3, a multiple-wavelength laser is proposed in which the lasing portion 201 of the system AlGaAs, the lasing portion 202 of the system AlGaInP, and the lasing portion 203 of the system GaN are formed side by side on one face of a substrate 231 made of SiC (silicon carbide) with isolation grooves 211a and 211b between them (refer to Publication of Japanese Unexamined Patent Application No. Hei-11-186651).
In the case of fabricating the monolithic type multiple-wavelength laser, however, there is a problem such that it is difficult to integrate lasing portions on the same substrate as one chip due to, for example, a large difference in lattice constant between the materials of the system GaN and the materials of the system AlGaAs or AlGaInP.
The hybrid type multiple-wavelength laser has, as already described, a problem of poor controllability on the light emitting point intervals. The side-by-side arrangement of three or more semiconductor lasers causes an inconvenience such that the controllability on the light emitting point intervals further deteriorates.